Target electrodes for use in television pick-up tube or the like



G. S. P. FREEMAN TARGET ELECTRODES FOR USE IN TELEVISION Jan. l 7, 1956PICK-UP TUBES OR THE LIKE Filed Oct. 22. 1949 i AVA GEORGE STANLEYPERCIVAL FREEMAN INVENTR.

HAS` AT TRNE Y United States Parent() TARGET ELECTRGDES FOR USE IYTELEVHSIN PICK-UP TUBE UR 'EEE LiKE George Stanley Percival Freeman,Chiswick, London,

England, assigner to Cinema-Television limited, London, England, aBritish company Application October 22, 1949, Serial No. 122,925

Claims priority, application Great Britain November 22, 1943 2 Claims.(Cl. 313-326) This invention relates to improvements in or relating totarget electrodes for use in television pick-up tubes or the like and ismore particularly concerned with such target electrodes of thedouble-sided type.

lt has previously been proposed to construct a target electrode, for usein a television pick-up tube, which comprises a thin metal ilm incontact with one face of a thin layer of insulating material. In theoperation of a tube comprising such a target electrode, primaryelectrons from a photosensitive cathode penetrate the metal lm andrelease secondary electrons in the insuiating material which areaccelerated away from the exposed face thereof. With such aconstruction, it is dicult to prevent primary electrons reaching themetal layer from passing through both the metal and insulating layers;such penetrating primary electrons, in conjunction with the releasedsecondaries, have a deleterious effect on the image signal when theinsulating face of the target is scanned with a low-velocity electronbeam and the signal is derived from the return beam. In another similararrangement, a thin metal film is juxtaposed to one face of a thin glasslayer; in this arrangement also there is the possibility that primaryelectrons incident on the metal lm may pass completely through thetarget electrode and appear at the exposed glass face where thesecondary electrons are liberated.

It is a primary object of the present invention to provide an improveddouble-sided target electrode, tor use in a television pick-up tube orthe like, which provides efficient image conversion while preventingprimary photoelectrons from entering the scanning section of the tube.

According to the invention there is provided a target electrode, for usein a television pick-up tube or the like, comprising the combination ofathin metallic layer, a thin semi-conducting layer and a thin insulatinglayer, capable of emitting secondary electrons, interposed between themetallic and semi-conducting layers.

The present invention avoids the difliculty hereinbefore mentioned byvirtue of the fact that electron velocities within the target electrodeare reduced so that it is irnpossible for primary electrons from thephoto-sensitive cathode to appear at the face of the target electrodewhich is scanned with the electron beam, although secondary emission iscaused at an intermediate interface of the target electrode.

The features of the present invention which are believed to'be novel areset forth with particularity in the appended claims. 'The invention,together with further objects and advantages thereof, may best beunderstood, however, by reference to the following description taken inconnection with the accompanying drawing in which:

Figure 1 is a sectional view, partly schematic, of a television pickuptube embodying a target electrode constructed in accordance with thepresent invention, and

Figure 2 is a sectional View of another embodiment of the invention.

With reference to Figure l, a television pickup tube comprises acontinuous light-transparent photo-cathode 10 formed on the inside ofthe window 11 of an evacuated envelope 12. A target structure 13 isdisposed in parallel relation with the photo-cathode 10 at a distance,for example, of about three millimeters. The target electrode 13comprises a very thin electron-permeable metallic lm 14 and a thin plate15 of semi-conducting material, constructed of glass for example, of thetype used ina conventional image orthicon. A thin layer 16 of insulatingsecondary electron emitting material is interposed between metallic tilm14 and semi-conducting plate 15; preferably, insulating layer 16 isalflxed to metallic lm 14, as for example, by depositing ilm 14 on layer16 in a manner well known in the art. Metallic lm 14 may conveniently bemade of aluminum, and insulating layer 16 may be constructed of aluminumoxide.;

On the other side of the target electrode 13 isdisposed an electron gun17, which may be of conventional construction, and an electronmultiplier systemv18 is arranged to intercept the return beam. Adecelerating'systern 19, which may comprise a series of annular metalrings operated at progressively higher potentials in a direction awayfrom electron gun 17, is provided, the purpose of which will behereinafter described. Scanning coils 20 are provided for deecting thecathode-ray beam to cause it to scan semi-conducting plate 15, and along solenoid 21 surrounds the entire tube envelope 12 to provide a longaxial magnetic field. Further electrodes 22, Z3, and 24 are provided tocontrol the cathode-ray beam; these electrodes may conveniently compriseconductive coatings on the internal walls of the tube envelope 12.

All of the electrodes may be supported within envelope 12 byconventional means well known in the art.

in operation, light from an image 25 is focussed on photo-cathode l@ bymeans of a lens 26, and primary electrons emitted from the photo-cathode1i? penetrate metallic tilrn 1d and come to restin the insulating layer16 just short of the surface thereof distant from the metallic ilm 14,thus releasing secondary electrons and imparting a local negative chargeto the semi-conducting plate 1S. Thus, a white picture point produces anegative charge on the scanned face of the semi-conducting plate 15,while a black picture point produces no change of potential. With thisarrangement, it is impossible for photoelectrons of secondary electronsto penetrate through the semi-conducting plate into the evacuated spacebeyond.

The scanning of the target electrode is performed in a manner similar tothe scanning in an orthicon or image ort'nicon type tube, but instead ofthe beam being decelerated to zero volts velocity, it is allowed tostrike the target with, for example, 205G volts velocity, thus releasingmore than one secondary electron per primary, and so restoring thetarget to an equiiibrium potential near that of the final anode. Thefact that the beam does not come to zero velocity improves the focus andalso reduces the tendency for the beam to defocus at regions of highnegative charge corresponding to high lights in the picture.

Since the envelope of the tube is surrounded by a long soienoidproviding an axial magnetic eid, the secondaries released by thescanning beam are substantially prevented irc-rn returning a rain orsecondaries the target, and consequently the tilt and bend signals areof negligible proportions, if they are present at all. Thesesecondaries, therefore, return to the gun end of the tube where theyenter the electron multipiier 13.

if the tube receives an excess of light on the photocathode, the targetmay travel so negative in potential that the picture disappears becauseof the inability of the scanning beam to reach it, but the range oftolerance is very much greater in a device constructed in accordancewith the invention than in the orthicon for instance, since thepermissible average potential change of the target is greater. Theremedy for such a disappearance Vof the picture is either to apply anegative pulse to the gun cathode, or to apply a positive pulse to thethin metal lm on the glass target. 'Y

To facilitate an understanding of the operation of the device, let it beassumed that point 27 of the image 25 is a black picture point, andpoint 2S isrra white picture point. When the image 25 is illuminated, alocal negative charge is imparted to t'he'point 29 on semi-conductingplate corresponding to point 28 of the image, and point 30 of thesemi-conducting plate 15, corresponding to point 27 ofV image 25,'isyunchanged in potential. Primary electrons from electron gun 17 scanningpoints 29 and 30.release secondary electrons from the surfaceV ofsemi-conducting plate lghowever, secondary electrons released frompoint'29 are accelerated to a greater velocity than those released frompoint 30. By adjusting the potentials applied to decelerating system 19,those secondary electrons released from black picture points may beprevented from reaching electron multiplier 1S, and are returned toacollecting electrode 31. On the other hand, secondary electrons releasedfrom a white picture point, being of Vhigher velocity, penetrate thedecelerating eld and enter electron multiplier 18.V The electronmultiplier may be of conventional construction, and a load irnpedance(not shown) may be coupled to the output electrode ofthe electronmultiplier to develop an output signal. y Instead of deriving thepicture signals from the return beam by way of the multiplier 18, thetube can be operated by drawing the picture signals capacitatively fromthe metal lm 14 on the glass target, using this as a signal plate,though the capacity to earth via the photo-cathode may be rather high.

In the preferred embodiment, an aluminum pellicle 14 is held paralleland in very close proximity to a very thin glass plate 15. The face ofthe aluminumnearest the glass plate 15 is coated with a layer 16 ofmaterialV having a high secondary emission coefficient.

The aluminum pellicle faces the photo-cathode, and

kthe photoelectrons, travelling athigh velocity, penetrate the aluminumfilm and are halted at the boundary of the high secondary emittingcoating, thus releasing a copious supply of secondaries. These strikethe glass plate,-which is about .0002 distant from the secondaryemitting coating so that the secondaries do not spread appreciably intravelling to the glass'plate. Thus va negative charge appears on theglass target where a light pattern exists on the photo-cathode.

In another embodiment, illustrated in Figure2, a glass plate 40 iscoated on one side with a cellular amorphous layer 41 of aluminum oxideby known methods, and an aluminum iilm 42 is produced on this oxidelayer as for example by methods similar to those used in producing ametal backing for a uorescent screen. The term amorphous as used inconnection with the aluminum oxide is used in its technical correctmeaning of having Vno crystalline structure, and not in the sense ofsignifying 4 spect than the target structure used in the conventionalimage orthicon. Y Y Y vA target electrode constructed in accordance withthe present invention has the following advantages as compared with thetype of target electrode used in the conventional image orthicon pickuptube.

(1) The full photo-current is utilized and there is not a 50% loss dueto the opaque grid wires in the conventional image oithicon target. (TheFigure 2 embodiment does not have this advantage for the reason stated.)

(2) The mechanism of releasing secondary electrons by halting primaryelectrons in theV surface boundary is very eiicient and so highmultiplication factors may be obtained. Y Y

(3) Since the glass target is in close proximity to the secondaryemitting surface, a volt or two potential diierence serves to saturatesecondary electron emission and so prevent the white afterj blackeifects noticeable in.

the image orthicon at high light levels. Hence the contrast range isgreater and there is lessrspurious shading due to redistribution ofelectrons overthe target at high lighting than in the image orthicon. 'Y

The main disadvantage of the present invention is that there is lessautomatic control of the glass target potential relative to thecapacitative electrode, and it may prove possible to drive the target sonegative that the scanning beam cannot reach it.Y However, this is not aserious drawback since, if such a condition arises, the fact that theglass target is driven negative with respect to the aluminum layer tendsto bias off the secondary emission caused by the photoelectrons and sohelps the scanning mechanism to resume operation efciently. Y

While particular embodiments of the present invention have been shownand described, it is apparent that Various changes and modifications maybe made, and it is therefore contemplated in the appended claims tocover all such changes and modifications as fall within theftrue spiritand scope of the invention.

I claim:

1. A target electrode for a picture-converting device comprising: athinl electroni-permeable aluminum lm; a thin semiconducting glass platesupported parallel and in close proximity to said lm; and a thin layerof insulating secondaryfelectron-emitting material spaced from saidplate and amxed to the surface of saidV lm facing said plate. Y

2. A target comprising: a thin metallic film; a thin plate ofsemi-conducting material; and a thin layer of insulatingsecondaryelectron-emitting material atxed to said iilm and spaced fromsaid plate, the spacing between said layer and said plate being of theorder of .0002 inch. v

References Cited in the le of this patent electrode forV apicture-converting device(

1. A TARGET ELECTRODE FOR A PICTURE-CONVERTING DEVICE COMPRISING: A THINELECTRON-PERMEABLE ALUMINUM FILM; A THIN SEMI-CONDUCTING GLASS PLATESUPPORTED PARALLEL AND IN CLOSE PROXIMITY TO SAID FILM; AND A THIN LAYEROF INSULATING SECONDARY-ELECTRON-EMITTING MATERIAL SPACED FROM SAIDPLATE AND AFFIXED TO THE SURFACE OF SAID FILM FACING SAID PLATE.